In the field of so-called “thermal” infrared detectors, it is known to use one-dimensional or two-dimensional arrays of elements sensitive to infrared radiation, capable of operating at ambient temperature.
A thermal infrared detector conventionally uses the variation of a physical quantity of a so-called “thermometric” or “bolometric” material, according to its temperature. Most currently, this physical quantity is the electric resistivity of said material, which is highly temperature-dependent. The unit sensitive elements of the detector, or “bolometers”, are usually in the form of membranes, each comprising a layer of thermometric material, and suspended above a substrate, generally made of silicon, via support arms having a high thermal resistance, the array of suspended membranes being usually called “retina”.
Such membranes especially implement a function of incident radiation absorption, a function of conversion of the power of the absorbed radiation into thermal power, and a thermometric function of conversion of the generated thermal power into a variation of the resistivity of the thermometric material, such functions being implementable by one or a plurality of distinct elements.
Further, the support arms of the membranes are also conductive and connected to the thermometric layer thereof, and means for sequentially addressing and biasing the thermometric elements of the membranes and means for forming electric signals usable in video formats are usually formed in the substrate having the membranes suspended thereabove. The substrate and the integrated means are commonly called “read circuit”.
The read circuit and the sensitive retina of a detector are usually integrated in a sealed package under very low pressure, provided with a window transparent to the radiation of interest, usually having a wavelength in the range from 8 to 14 micrometers. This range corresponds to the transparency window of the atmosphere and to the majority of the radiations originating from scenes in the vicinity of 300 K.
To obtain a thermal or pyrometric image via such a detector, the scene is focused through an adapted optical system onto the focal plane having the retina arranged thereon, and clocked electrical stimuli are applied via the read circuit to each of the bolometers, or to each row of such bolometers, to obtain a “video” electric signal forming the image or the measurement of the temperature reached by each of said elementary detectors. The signal may be more or less elaborately shaped, directly by the read circuit, and then transmitted in analog or digital form to an electronic system external to the package. This electronic system typically applies various corrections to each video frame delivered by the detector, in particular a correction of spatial offset and gain dispersions (called “NUC” for “Non Uniformity Corrections”), to generate a thermal or pyrometric image capable of being displayed, or more generally for the use of the signals thus formed from the observed scene.
The average temperature of the sensitive membranes is essentially imposed by the substrate temperature which, in the absence of stabilization, varies due, in particular, to changes in environmental conditions, which are essentially reflected by thermal conduction through the integrated system elements. Such temperature variations induce a drift in the average signal at the output of the bolometers. Bolometric detectors may be equipped with a module for stabilizing the substrate temperature, usually a Peltier-effect module (TEC, for “Thermo Electric Cooler”) to avoid such signal drifts. Such stabilizing means however make the component more complex and expensive and imply an electric power consumption which is all the higher as the ambient temperature is distant from the selected stabilization temperature.
As a variation, the detector comprises no temperature stabilization module, and an element for compensating the focal plane temperature (TPF) is provided in the electronic signal forming circuit in relation with the temperature of the bolometers, said compensation element being itself bolometric, that is, having its electric behavior following the substrate temperature, but remaining essentially insensitive to radiation. This result is for example obtained by means of bolometric structures provided, by construction, with a lower thermal resistance towards the substrate, and/or by masking these structures behind a screen opaque to the thermal radiation to be detected. The use of such compensation elements further has the advantage of eliminating most of the so-called common-mode current originating from imaging or “active” bolometers.
FIG. 1 is an electric diagram of a bolometric detector 10 with no temperature regulation, or “TECless” detector of the state of the art, comprising a common-mode compensation structure, and FIG. 2 is an electric diagram of a circuit used to form a read signal of a bolometer of the compensated common-mode detector. Such a detector is for example described in document: “Uncooled amorphous silicon technology enhancement for 25 μm pixel pitch achievement”; E. Mottin et al, Infrared Technology and Application XXVIII, SPIE, vol. 4820E.
Detector 10 comprises a two-dimensional array 12 of unit bolometric detection elements 14, or “pixels”, each comprising a sensitive resistive bolometer 16 in the form of a membrane suspended above a substrate, such as previously described, and having an electric resistance Rac. Each bolometer 16, also called “active” or “detection” bolometer, is connected at one of its terminals to a constant voltage VDET, especially the ground of detector 10, and at its other terminal to a MOSFET biasing transistor 18 operating in saturated state, for example, an NMOS transistor, setting voltage Vac across bolometer 16 by means of a gate control voltage GAC. Pixel 14 also comprises a selection switch 20, connected between MOS transistor 18 and a node A provided for each column of array 12, and driven by a control signal SELECT, enabling to select bolometer 16 for the reading thereof. Transistor 18 and switch 20 are usually formed in the substrate under the influence of the membrane of bolometer 16. Elements 16 and 18 form a so-called “detection” branch.
Detector 10 also comprises, at the foot of each column of array 12, a compensation structure 22, also usually called “skimming” structure. Structure 22 comprises a compensation bolometer 24, of electric resistance Rcm, made insensitive to the incident radiation originating from the scene to be observed.
Bolometer 24 is built by means of the same thermometric material as bolometer 16, but according to a structural configuration provided with a very low thermal resistance towards the substrate. This result can be easily achieved, for example, by means of a direct construction of the resistive elements of the bolometer in contact with the substrate, or by the simple absence of arms of a resistive bolometric structure, which is however suspended, or also by preserving a thermally-conductive material between the substrate and the bulk of the compensation bolometer. The electric resistance of bolometer 24 is thus essentially dictated by the substrate temperature, bolometer 24 then being said to be “thermalized” to the substrate.
Bolometer 24 is connected at one of its terminals to a constant voltage VSK, and compensation structure 22 further comprises a MOSFET bias transistor 26 operating in saturated state, having a biasing opposite to that of transistors 18 of detection pixels 14, for example, a PMOS transistor, setting voltage Vcm across bolometer 24 by means of a gate control voltage GCM and connected between the other terminal of compensation bolometer 24 and node A. Elements 24 and 26 form a so-called compensation terminal common to each column. While it is not necessary to adjust resistance Rcm of bolometer 24, by design, to a value close to that of bolometer 16, it is however necessary to adjust the current flowing therethrough to a value close to that which runs through the detection branch during the reading. This result is for example and typically obtained by means of a resistance lower than that of bolometer 16, and of a bias voltage Vcm smaller roughly by the same proportion.
Detector 10 also comprises, at the foot of each column of array 12, an integrator 28 of CTIA type (“Capacitive TransImpedance Amplifier”), for example comprising an operational amplifier 30 and a capacitor 32 connected between the inverting input and the output of amplifier 30. Its inverting terminal and its non-inverting terminal are further respectively connected to node A and to a constant voltage VBUS. Voltage VBUS thus forms a reference for the output signals, and is between VDET and VSK. A switch 34, driven by a signal Reset, is also provided in parallel with capacitor 32, for the discharge thereof. The outputs of CTIAs 28 are eventually, for example, connected to respective sample-and-hold circuits 36 for the delivery of voltages Vout of CTIAs in multiplexed mode by means of a multiplexer 38 to one or a plurality of series output amplifier(s) 40. It may also be integrated at the output of the digitizing means (analog-to-digital conversion: ADC). Finally, detector 10 comprises a synchronization and video processing unit 42 particularly controlling the different previously-described switches. In operation, array 12 is read from row by row, the whole of read rows thus forming an image frame, or “frame”. To read from a row of array 12, switches 20 of the line of pixels 14 are turned on and switches 20 of the other lines are turned off.
After a phase of discharge of the CTIA capacitors at the foot of the columns, achieved by the turning-on of switches 34 followed by their turning-off, a circuit such as shown in FIG. 2 is thus obtained for each pixel of the row being read from. A current Iac flows through detection bolometer 16 of the pixel under the effect of its voltage biasing by MOSFET transistor 18, and a current Icm flows through compensation bolometer 24 of the compensation structure under the effect of its voltage biasing by MOSFET transistor 26. These currents are subtracted from each other at node A, and the resulting current difference is integrated by CTIA 28 during a predetermined integration period Tint. Output voltage Vout of CTIA 28 thus is a measurement of the variation of the resistance of detection bolometer 16 caused by the incident radiation to be detected since the non-useful part of current Iac is at least partly compensated for by current Icm specifically generated to reproduce this non-useful part.
Further, as previously described, since detection bolometers 16 are not strictly identical, voltages Vout exhibit a dispersion of their values in front of a uniform scene, or “offset dispersion”. Similarly, a dispersion of the detector responsiveness, or “gains dispersions”, that is, a dispersion of the variations of voltages Vout in front of a uniform variation of a uniform scene, can be observed. Such dispersions adversely affecting the quality of the images generated by the detector, voltages Vout are usually corrected by unit 42 by at least the offset dispersion. The correction of the offset dispersion only is usually designated as a “1-point” correction, while the correction combining both the offset dispersion and the gain dispersion is currently called “2-point” correction.
In the following, expression “raw signal” designates a non-corrected signal, and particularly designates signals upstream of unit 42. In particular, this expression indifferently designates analog signals, for example, voltages Vout at the output of the CTIAs, or digital signals, for example, digital values of voltages Vout if the “1-point” or “2-point” correction is implemented digitally, the analog or digital form of the raw signals being easily understood from the context in which the expression is used. Expression “raw image” or “raw frame” thus designates all the raw signals originating from a reading from array 12.
Similarly, expression “corrected signal” designates a signal having undergone a correction aiming at eliminating or compensating for the signal dispersion, for example, a “1-point” or “2-point” correction, and expression “corrected image” or “corrected frame” thus designates an image or frame corrected from the raw values by the “1-point” or “2-point” correction.
A defective pixel is a unit detection element having its corresponding signal, for example, voltage Vout in the above-described detector example, considered as non usable to form an image element representative of the observed scene. For example, a defective pixel produces a signal which is saturated on one side or the other of the output dynamic range of the signal-forming chain and this, whatever the observed scene.
More generally, a defective pixel is a pixel which may generate, after the application of a “1-point” or “2-point” correction, a signal having an abnormal deviating to its neighbors, which cannot be imputed to the observed scene and is visible on an image or on a video sequence in given environmental conditions. This deviating may originate from various physical causes which induce a marked deviation of the intrinsic electric and/or optical behavior of the defective pixel with respect to its neighboring pixels, considered as “normal”. “Normal” behavior of a pixel means a pixel behavior close to that of the set of pixels of the retina or of the set of signal-forming chains, so that a “1-point” or “2-point” correction shows no deviating of the corrected signal originating from the pixel in standard operating conditions.
The following table lists known recurrent defects associated with bolometric retinas as well as the physical causes thereof. Notation CL (for continuous level) here refers to the value of the raw signal originating from a pixel when the retina is exposed to a substantially temperature-uniform scene.
Physical cause of the defectEffect/Defect observed on the CLpixelShorted pixelCL at the dynamic range limitOpen-circuit pixelCL at the dynamic range limitAbnormal deviating of theAbnormal CL deviating for a strongresponsiveness of the pixel signal-incident flowforming chainAbnormal deviating of theAbnormal CL deviating whatever thebolometer resistanceincident flowAbnormal noise deviatingFluctuating CL for a constant flowPixel coming after (in read mode) aFluctuating CL whatever the incidentpixel which is shorted or in openflow and abnormal CL deviatingcircuit. This pixel is located on thewhatever the incident flowsame row or on the same columnto within a few indexes (i, j) andhas its signal processing chaintemporally affected by an abnormalstate of the previous signal.Electronic defect on the pixelFluctuating CL whatever the incidentsignal-forming chain (for example,flow and CL deviating independentdefect of the bias transistor or offrom the flow or from the bolometerthe CTIA)
To know the existence of possible defective pixels, operability tests are usually carried out in factory, that is, before the detector is put into service, by performing various measurements and analyses aiming at establishing the compliance of the measurements of each of the pixels with respect to a predetermined functional compliance window. The concerned measurements generally comprise extracting, at the level of each of the pixels, especially the following functional parameters:                continuous level CL;        responsiveness Resp, that is, the signal variation originating from the pixel with respect to a uniform variation of the uniform scene to which the retina is exposed; and        the temporal noise of the signal originating from the pixel.        
Operability tests thus enable to establish a list of defective pixels, or “operability table”, that is, a series of row and column coordinates defining the location of said pixels in the pixel array. The operability table is then stored in unit 42 and exploited by said unit on use of the detector to correct the points of the images corresponding to the defective pixels, for example, the replacing of the signal of these pixels by an average of the signals provided by the neighboring pixels.
However, an operability table is the image of the detector defects at a specific time only, and for specific environmental conditions, particularly of temperature and illumination, and for specific operating conditions, particularly biasing conditions. First, a pixel may be defective for the specific environmental and operating conditions implemented for the operability test and be however considered non defective for other environmental and operating conditions. Thus, a modification of the environmental and operating conditions is capable of inducing on certain pixels initially considered compliant, modifications of their output signal considered as non-compliant in the new environmental and operating conditions, thereby making the operability table irrelevant for these pixels. To overcome such a problem, operability tables should thus be determined for multiple environmental and operating conditions. In addition to the cost and the duration necessary to obtain a multiplicity of tables, it is not possible to reproduce during initial factory operability tests all the conditions that a detector can encounter during its use, so that the issue of the validity of embarked tables regarding the conditions encountered by the detector would arise even so.
Further, for specific environmental and operating conditions, the behavior of certain pixel may unexpectedly substantially drift, and certain pixels may even totally lose their functionality. The operability table may thus prove being irrelevant once the detector is in service. The operability table should thus be regularly updated to take this phenomenon into account. Now, the above-described measurements usually require using complex and expensive optical devices, particularly using reference “black bodies” for different temperatures. Similarly, the bolometer resistance is not directly measurable from the output signals of the signal-forming chains due to the fact that a compensation is generally provided to reject the common mode in the forming of these signals to only measure small signal variations due to the resistance variations induced by the incident flow of the observed scene.
The determination of the bolometer resistances thus requires specific procedure and equipment. Since only the constructor generally has this type of equipment, the updating of the operability table accordingly requires returning the detector to the factory to implement a new operability test.